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Retirement Benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 25, 2021
Jun. 26, 2020
Jun. 25, 2021
Jun. 26, 2020
Pension        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2,724 $ 2,193 $ 5,445 $ 4,615
Interest cost 2,976 3,227 5,957 6,634
Expected return on assets (5,131) (4,651) (10,261) (9,401)
Amortization and other 2,555 2,517 5,106 5,238
Net periodic benefit cost 3,124 3,286 6,247 7,086
Postretirement Medical        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 175 130 350 305
Interest cost 250 260 500 508
Amortization 250 190 500 367
Net periodic benefit cost $ 675 $ 580 $ 1,350 $ 1,180